1. Field of the Invention
This invention relates to a light beam scanning recording apparatus for recording information on a recording material by scanning the recording material with a light beam. This invention particularly relates to a light beam scanning recording apparatus for recording a continuous tone image by using a semiconductor laser as a light beam generation means.
2. Description of the Prior Art
Light beam scanning recording apparatuses wherein a light beam is deflected by a light deflector and scanned on a light-sensitive recording material for recording information on the recording material have heretofore been used widely. As one of the means for generating a light beam in the light beam scanning recording apparatuses, a semiconductor laser is used. The semiconductor laser has various advantages over a gas laser or the like in that the semiconductor laser is small, cheap and consumes little power, and that the laser beam can be modulated directly by changing the drive current.
FIG. 4 is a graph showing the relationship between the optical output and the drive current of the semiconductor laser. As shown in FIG. 4, the optical output characteristics of the semiconductor laser with respect to the drive current change sharply at a drive current value. Therefore, it is not always possible to apply the semiconductor laser to recording of a continuous tone image. Specifically, it is difficult to control the optical output of the semiconductor laser over the point at which the optical output characteristics change sharply. Also, when intensity modulation is conducted by utilizing only the region on the upper side of the sharp change point wherein the characteristics are linear, it is possible to obtain a dynamic range of the optical output of only approximately 2 orders of ten at the most. As is well known, with a dynamic range of this order, it is impossible to obtain a continuous tone image having a high quality.
Accordingly, an attempt has been made to obtain a continuous tone image by maintaining the optical output of the semiconductor laser constant, continuously turning on and off the semiconductor laser to form a pulsewise scanning beam, and controlling the number of pulses for each picture element to change the scanning light amount.
However, in the case where pulse number modulation as mentioned above is conducted and a resolution of the scanning light amount of 4 orders of ten is to be obtained when the picture element frequency is 100 kHz, the pulse frequency must be adjusted to be as high as 1 GHz. Though the semiconductor laser itself can be turned on and off at such a high frequency, a pulse counting circuit or the like for control of the pulse number cannot generally be operated at such a high frequency. As a result, it becomes necessary to decrease the picture element frequency to a value markedly lower than the aforesaid value. Therefore, in the case where a continuous tone image is recorded by pulse number modulation, the recording speed becomes very low.